FIG. 1 represents a conventional magnetic random access memory (MRAM) cell 1. The MRAM cell 1 comprises a magnetic tunnel junction 2 formed from a first ferromagnetic layer 21, a second ferromagnetic layer 23 and a tunnel barrier layer 22 having a junction resistance-area product RA. In the example of FIG. 1, the MRAM cell is intended to be written using a thermally assisted (TA) write operation and the magnetic tunnel junction 2 further comprises a second antiferromagnetic layer 25 exchange-coupling the second ferromagnetic layer 23. During the write operation, a heating current 32 can be passed, via a current line 4, in the magnetic tunnel junction 2 such as to heat the magnetic tunnel junction 2 at a high temperature threshold at which a magnetization of the storage can be freely switched. The first ferromagnetic layer 21 can have a magnetization being free to switch or also be exchanged-coupled by a first antiferromagnetic layer 24 such as to have a fixed magnetization.
The tunnel barrier layer 22 is often made from a magnesium oxide (MgO) layer. Indeed, large tunnel magnetoresistance (TMR) for example of up to 200% can be obtained for the magnetic tunnel junction 2 comprising a crystalline MgO-based tunnel barrier layer 22. Such tunnel barrier layer 22 made of MgO can be obtained by using an RF magnetron sputtering method. However, the MgO formation method by the RF magnetron sputtering can give rise to dispersion in normalized tunnel resistive value (RA) and possible deterioration of the yield factor at the time of device fabrication.
In U.S. Pat. No. 6,841,395, the MgO barrier layer is formed by a method comprising the steps film formation of a metal Mg layer, forming oxygen-doped metal Mg layers, and bringing the laminated layers into an oxidation process. However, during the step of oxidizing the Mg layer, defects such as pinholes can be formed on the MgO layer surface. Defects formation can arise due to the fact that the MgO oxide has a larger volume than metallic Mg. As a result, current leakage may occur yielding to a lower resistance and a lower breakdown voltage of the MgO tunnel barrier 22, especially for low RA values, below 50 ohm□m2. Such current leakage can occurs when a current is passed in the magnetic tunnel junction 2 for heating the magnetic tunnel junction 2 during the TA write operation of the MRAM cell 1, or for reading the junction resistance during a read operation of the MRAM cell 1. The presence of defects can thus decrease the resistance of the MgO tunnel barrier 22, and the tunnel magnetoresistance TMR of the magnetic tunnel junction 2 comprising such MgO tunnel barrier 22 is also lowered. Moreover, a lower breakdown voltage of the barrier layer 22 can be observed.
Reducing the effect of pinholes requires having a relatively thick Mg layer and/or growing relatively thick oxide layers. Increasing the thickness of the MgO tunnel barrier layer may 22 can yield a RA that is too large so that the voltage for driving the magnetic tunnel junction device becomes too high. Also, if the initial Mg layer is too thick a single step oxidation is not oxidize completely this Mg layer. The Mg layer will then be under oxidized, with lower RA, lower TMR and lower breakdown voltage.